Read about how cracks occur in 3-coat stucco systems and some of the best ways to prevent them.
Over the last few years, stucco has become a top exterior finish choice for housing giants. However, along with its popularity have come several issues typically caused by cutting corners during the production schedule to save time. As we all know, cutting corners is both risky and costly, and it can hurt a reputation, especially because homes in a community are in full public view, day in and day out. So it's imperative to arm yourself and your construction teams with the right knowledge and quality assurance procedures to ensure that every stucco finish is installed correctly.
Stucco is well-known as a durable, versatile and cost-effective exterior finish for homes. And indeed, when installed correctly, it holds up well under even the extreme conditions of areas like Alaska and Florida. Although stucco is most prevalent in the southwestern U.S., its durability has been proven in all climates, whether hot, cold, dry or wet. Its versatility is best seen when stucco is applied to any flat or curved surface on the outside of a home, such as on architectural details around a window or door, lending style to a home's façade. Stucco also has a low first cost and typically requires a relatively small amount of maintenance, making it a cost-effective exterior finish.
There are several different types of stucco installations. The most traditional and popular of these types is 3-coat stucco, in which stucco is applied in three coats over metal reinforcement. This stucco system consists of a scratch coat, brown coat and finish coat. The scratch coat is applied first to provide a strong base for the system. It's embedded in metal lath, which strengthens and secures the coat. The brown coat is applied next to create an even surface for the finish coat. The finish coat is applied last, creating a decorative texture on the wall surface.
Cracking is one of the largest complaints from homeowners with stucco homes. All stucco cracks are the result of stucco releasing stresses placed on it by shrinkage, expansion or other movement. Depending on size, cracks can run the gauntlet from invisible to unsightly to the cause of major water intrusion, if they aren't addressed in a timely manner.
Usually, stucco releases stress through a series of micro cracks spread over the wall surface or at predetermined locations where control joints are placed. Rich Baker, a building performance specialist with Ibacos Homebuilding Quality Consulting in Pittsburgh, suggests placing control joints about every 144 square feet. "Try aligning them with windows and doors or evenly between floors in order to be more aesthetically pleasing," he says. Baker has assisted housing's giants with identifying, correcting and preventing construction quality issues related to stucco. Micro cracks are a normal occurrence and are mostly invisible to the naked eye.
Small cracks that are less than 1/16 inch are common and can be easily repaired with minor filling and repainting, if needed. This level of maintenance is expected with stucco homes, and these cracks don't undermine the integrity of the exterior finish.
Large cracks that are more than 1/16 inch can cause water intrusion issues and other damage. Large cracks are typically the result of errors made during the stucco installation that cause the stucco to store stress until it's forced to crack. These cracks can run deep, exposing the building materials beneath the stucco, such as housewrap, to the elements. Because these materials aren't built to withstand prolonged exposure, water will eventually penetrate them and move toward the interior of the home. In extreme cases, these larger cracks can also shear the metal lath, which places more stress on the rest of the stucco system, potentially causing additional cracking.
Some of the most common errors that cause stucco to crack are relatively easy to prevent. Let's look at three.
After proper hydration, a pH indicator strip demonstrates that the pH of this wall is around 9, which is an acceptable level. The ph level should be no higher than 10 when proceeding to the next coat.
Stucco mix consists of three main ingredients: cement, sand and water. The mix proportions are carefully controlled to create a stucco mix that, when adequately hydrated, allows each coat to relieve stresses through a dispersed pattern of micro cracks prior to the application of the next coat. Out of the three ingredients, there's a greater opportunity for error when it comes to the sand. When there isn't enough sand in a coat, the cement paste shrinks too much. The excessive shrinkage puts too much stress on the coat, leading to deep cracks on the surface of the wall. These cracks frequently occur at locations where the cross-section of the stucco is reduced, such as at a window. When there's too much sand in the mix, water has to be added to maintain workability, which reduces the water-to-cement ratio of the mix, weakening the stucco. If too much water is added, the stucco loses cohesion and slumps on the wall.
To prevent this issue, the scratch and brown coats have to be mixed according to the manufacturer's instructions. According to ASTM standards, the brown coat should have slightly more sand than the scratch coat. "Sand is typically added to the stucco mix by the shovel full," Baker says. "While it may sound trivial, it's important that everyone on-site uses the same size and shape shovel in order to get a consistent mix from batch to batch." Enough sand has to be added for shrinkage resistance while not affecting the stucco's workability.
It's important to apply the scratch and brown coats at consistent thicknesses over the surface of the wall. When a coat is inconsistent, for instance, ¼-inch thick in one area and ¾-inch thick in another, its strength varies. Thin areas aren't strong enough to resist cracking. Achieving a
The thickness of the stucco on this wall is inconsistent, exposing the metal lath.
uniform thickness can be particularly difficult with open-frame construction; if sheathing is installed only at corners or above and below windows, it creates an uneven surface for applying the stucco. As a result, the stucco is likely to be thinner over studs and thicker across stud bays. Stucco is particularly susceptible to cracking around windows and trim boards, where it's applied thicker to create architectural character.
The scratch coat should be applied at a uniform thickness of 3/8 inch to ½ inch and be embedded well in the metal lath. Baker tells Housing Giants, "It's important to the strength of the stucco to fur the metal lath out from the wall, allowing it to be embedded in the scratch coat." The brown coat should be applied at a uniform thickness of ¼ inch to 3/8 inch, except at architectural details, where it can be thicker. The brown coat should be thinner than the scratch coat, so the scratch coat is the strongest layer.
After a coat of stucco is applied, it should be hydrated to allow the stucco to cure to full strength. Once the coat has chemically reacted with a sufficient amount of water and is allowed to dry, it reduces in volume and shrinks. The shrinkage builds up stresses in the coat, which are released as a series of micro cracks. These cracks aren't deep enough to undermine the integrity of the stucco system and are covered up by the next coat. In contrast, when a coat is insufficiently hydrated, it can't cure properly. The coats act as a single coat, placing greater stress on the system and causing larger, deeper cracks to form. For example, if the brown coat is applied to a scratch coat that hasn't sufficiently cured yet, it will cause the scratch coat to absorb water from the brown coat and continue to cure. The two coats, which are curing, drying and shrinking at the same time, will act as one coat, doubling the amount of stress they place on the stucco system. The excessive stress is released as deep cracks, which may run through both coats and even shear the metal lath.
To prevent insufficient hydration, the answer is simple: adequately hydrate each of the coats. To hydrate a coat, the wall surface should be flooded three times with a gentle stream of water that moves from the bottom of the wall to the top until the water runs off in sheets. The second and third passes on the wall require significantly less water to reach the sheeting stage, because with each pass, the stucco will accept less water. Each exterior wall should be hydrated at least once, but up to as many as three times if the wall continues to absorb water. The stucco should retain moisture for the first 48 to 72 hours after it's applied to cure to full strength.
Even though stucco is versatile, durable and cost-effective, it isn't a simple process to install it correctly. However, it remains a great material to use as an exterior finish, as long as your construction teams have the right knowledge and right quality assurance procedures in place. Making sure your homes' stucco finishes are installed correctly will help you keep those hundreds or even thousands of dollars per home in warranty callbacks in your pocket.
Michael Dickens is the CEO of BuildIQ, which provides online information, tools and training in home building best practices to help builders take their homes and businesses to the next level of quality and performance.
For more information about stucco, the Portland Cement Association offers a how-to guide and technical manual with in-depth descriptions of stucco installations. You can purchase it on PCA'S website.
- Lack of control and expansion joints
- Poor sheathing installation
- Incorrect application of metal lath